1. Field of the Invention
The present invention relates to an image compressing method and an image compression apparatus for lossless compressing image data obtained from an image pickup device.
2. Description of the Related Art
Electronic cameras have become remarkably widespread, but a number of them reserve the number of captured records by lossy compressing image data due to the restrictions on the recording capacity of an image record medium.
There also is a recording system for enhancing the quality of playback images of image data by increasing the recorded data capacity per image while reducing a compression rate or performing a non-compressing process. One of the recording systems is a RAW image data recording system. The RAW image data recording system omits a series of digital image processing operations in an electronic camera, A/D converts the output of an image pickup device, and then digitally record the converted data. The RAW image data have required a larger image recording capacity per image with an increasing pixels required for an image pickup device. Therefore, there is a strong demand for reducing the requirements for the image recording capacity without degrading the quality of playback images by lossless compressing the RAW image data.
If data values are locally distributed in a process area when image data is lossless compressed, then the compression rate (coding efficiency) can be enhanced in entropy coding. Since adjacent pixels have different color components in the RAW image data output from an image pickup device to which a Bayer filter is applied, the correlation between adjacent pixels is rather small in most cases. Therefore, it has not been easy to raise the compression rate when Bayer RAW image data is lossless compressed.
On the other hand, an apparatus for lossless compressing and recording CCD-RAW data is disclosed by, for example, Japanese Published Patent Application No. 2001-60876, Japanese Published Patent Application No. 2001-61067, Japanese Published Patent Application No. 2001-326939. Japanese Published Patent Application No. 2002-171531 also discloses the configuration in which a pixel signal of a solid-state image pickup device is separated for each pixel, a difference signal or a ratio signal between a reference signal and another signal is obtained, and a data file is generated and recorded from the difference signal or the ratio signal between the reference signal and the other signal. Additionally, Japanese Published Patent Application No. 2003-125209 discloses the apparatus including a compressing unit for separating and extracting Bayer RAW image data for each color component, and performing a series of compressing processes for each color component.